The present invention relates to a laser printer, copier, facsimile transceiver or similar electrophotographic image forming apparatus and, more particularly, to an electrophotographic image forming apparatus of the type having a transfer medium in the form of, for example, a transfer belt.
Conventional image forming apparatuses include one which transfers a toner image from an image carrier to a transfer medium, e.g., a transfer belt by generating an electric field between the image carrier and the belt and then transfers it to a paper sheet or similar transfer material. A prerequisite with this type of apparatus which transfers a toner by an electrostatic force is that an electric field for transfer having a certain value be stably developed in a transfer region. In practice, however, the resistance of the transfer medium changes due to aging and environmental conditions since it is formed of a resistor. To eliminate this problem, the transfer belt may be implemented as an insulating belt, as disclosed in, for example, Japanese Patent Laid-Open Application No. 57364/1985. In this case, the potential of the belt is measured before and after image transfer while transferring means (e.g. transfer charger) is so controlled as to maintain the belt potential constant.
The insulating belt scheme stated above stabilizes image transfer overcoming changes in the electrical characteristics of the transfer medium by controlling the transferring means. However, since the location for measuring the belt potential is remote from the transfer region, the potential deposited on the belt in the transfer region will have been attenuated to some degree when measured by, for example, an electrometer located downstream of the transfer region. In an ordinary condition, the attenuation does not vary beyond a certain range and, therefore, has only to be estimated. However, as the surface of the transfer medium adsorbs moisture in a humid environment, for example, the attenuation increases due to leaks and other similar causes and, in addition, changes with the degree of moisture adsorption. In such a condition, it is extremely difficult to execute accurate control over the transferring means. In light of this, there has been proposed an image forming apparatus which uses a transfer medium in the form of a belt having a resistance of about 10.sup.9 ohms in order to guarantee a sufficient nip width for transfer and to generate an electric field for transfer efficiency. In this type of apparatus, the image carrier is disposed between a plurality of electrodes which apply transfer voltages to the belt. The transfer voltages from the electrodes generate an electric field in the transfer region where the image carrier and transfer medium, i.e., belt contact each other.
However, by examining a relation between the transfer voltage to be applied to each electrode and the transfer efficiency, it was found that an optimal transfer voltage is not achievable even with the above-described type of apparatus due to aging and environmental factors although it is optimal at first. Specifically, despite that the same transfer voltage is applied to the transfer medium, the same electric field cannot be developed in the transfer region where the transfer medium contacts the image carrier, for the following reasons:
(1) A toner melts and sticks fast to the surface of the transfer medium as the apparatus is repetitively operated; and
(2) The resistance of the transfer medium changes due to the influence of ambient conditions (especially humidity).
Moreover, the above occurrences are observed not only over the entire transfer medium but also in limited part of the transfer medium. Specifically, when carbon or similar resistance control agent used to regulate the transfer medium to the above-mentioned particular resistance is dispersed nonuniformily, a current flows in some part and does not flow in some other part. Then, the deterioration of the transfer medium due to aging is not uniform, aggravating the local scattering. The resulting image would suffer from noticeable irregularity.